Four-wheel drive systems are designed to provide both two-wheel drive and four-wheel drive as needed or desired. The four-wheel-drive system, in addition to the conventional rear two-wheel drive, requires the following additional components: a transfer case mounted at the rear of an automatic or manual transmission; a front-drive differential and axles; a front drive shaft; and front-drive-wheel locking hubs.
Two basic types of four-wheel-drive designs are used-part time and full time. With the full-time four-wheel drive, the driver can select two- or four-wheel-drive from the driver's seat. This invention relates to a four-wheel-drive vehicle which selectively transfers between two- and four-wheel-drive.
As shown in FIG. 1, a button type part time four wheel drive has a button assembly 10 for selecting driving type; a controller 20, i.e., an electric control unit (hereinafter "ECU"); and a transfer case 30 activated by solenoids 40 and actuators 50 controlled by the ECU 20. Reference number 70 indicates a power supply operated by engine negative pressure.
The transfer case 30 is connected to the transmission 60 which enables the engine 80 to propel the vehicle under various loads and has a fixed connection with one of the drive axles 61 or 62 and selectively meshes with the other drive axle 62 or 61, respectively, by the activation of the actuators 50 and solenoids 40.
The ECU 20 determines transfer from 4-wheel drive to 2-wheel drive or vice versa regarding vehicle speed. That is, ECU permits a specific transfer only when the vehicle speed which can be informed by a speed sensor 100 is below a predetermined value.
The ECU 20 for transferring drive mode among 4*4H (four wheel drive-high speed), 4*4L (four wheel drive-low speed), and 4*2H (two wheel drive-high speed) is controlled by a button assembly 10.
Since transfer is designed to be achieved even while a clutch (not shown) between the engine 80 and the transmission 60 is being engaged, drivers experience a transfer shock, especially transferring the drive type from 4*2H to 4*4H, which is because speed limit for transferring from 4*2H to 4*4H is higher than for transferring between 4*2H and 4*4L, and between 4*4L and 4*4H.
That is, according to the conventional transfer logic the ECU 20 permits transfer from 4*2H to 4*4H even when vehicle speed is relatively high.